Howdy,I am interested in constructing features such as rocky walls, caves, even simulations of tree roots including terraced structures in a freshwater tank. Kind of like making a 3-D backwall but with areas for plants, driftwood, etc. I've received mixed reports on the use of styrofoam and concrete (without lime). Even making a mound in the tank seems to be difficult, as the gravel or sand will, over time, slip downward.Any help from wise folks?Thanks,Paul

Styrofoam covered with potable water concrete product like the Sika products is aquarium safe, unless you have a really aggressive Pleco that can rasp through a thin layer of concrete.

Expanding foam filler, sold as construction material is aquarium safe, but is rather garish in color. There is a black form sold for ponds that is much better for color, but a little weird to work with. Gets the job done, but be prepared to scrape some off and redo.

There's an expanding foam product called "Great Stuff" (found at hardware stores or even Walmart) that can work well as long as you give it plenty of time to cure and also have enough rocks or a design that will prevent it from floating.

Krylon fusion spraypaint is aquarium-safe and can be used to give it a more natural look.

Hey, thanks a heap for the suggestions and observations.I've located some harder-than-usual styrofoam that is slate gray in color locally. This should be much easier to carve and of course look much better if pieces break off. I'm trying to understand why cement and other materials are used to cover the styrofoam. Does the styrofoam deteriorate over time under water?I can understand painting the styrofoam to give it the look of nature but is the paint necessary to somehow coat the styrofoam?Also, how expensive is it to coat the styrofoam in fiberglass as compared with painting with the paint mentioned above?Thanks very much,Paul

I think the main reasons are to make the styrofoam more natural looking plus help weigh it down/keep it from floating.

Most styrofoam by itself shouldn't degrade in water and should be safe. IDK about your grey styrofoam, though- might not hurt to put some in a bucket of water and test things for a while before jumping in with both feet.

you can build up hills using small pieces of thin material stuck into the gravel like a dike and then pour on the next layer until it falls over the edges of the dike to make a smooth "hill" and build upwards like that, you can also use egg crate to lay on the gravel surface and then build upwards to the next egg crate piece cut to match the size of hill you are trying to make.

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